Heretofore, as emulsifiers for emulsion polymerization, anionic surfactants such as soaps, sodium dodecylbenzene sulfonate, polyoxyethylene alkyl phenyl ether sulfate ester salts, and polyoxyethylene alkyl ether sulfate ester salts; and nonionic surfactants such as polyoxyethylene nonyl phenyl ethers and polyoxyethylene alkyl ethers have been used. However, a polymer film obtained from a polymer dispersion using any of the above emulsifiers has problems that the emulsifier used remains in a free form in the polymer film, and therefore, the water resistance and the adhesiveness of the film are poor, etc. Therefore, as measures for the problems, a lot of reactive emulsifiers having a copolymerizable unsaturated group have been proposed (for example, PTL 1 to PTL 3).
A reactive emulsifier having an acrylic group or a methacrylic group as a copolymerizable unsaturated group, which has been proposed in the prior art, has high copolymerizability with a monomer, but has a problem that the polymerization stability during emulsion polymerization is deteriorated. For example, agglomerates during emulsion polymerization are increased, particles formed by emulsion polymerization are coarse and have poor stability over time, and so on. A reactive emulsifier having an allyl group as a copolymerizable unsaturated group sometimes has poor copolymerizability between the reactive emulsifier and a monomer depending on the type of the monomer or the polymerization conditions. Also a polymer film obtained from a polymer dispersion has a problem which remains unsolved that a film having quite satisfactory water resistance and adhesiveness cannot be obtained or a processing trouble is caused due to foaming in the polymer dispersion. In particular, in the case where styrene is contained as a monomer during emulsion polymerization, the above-described problems often occur, and it has been strongly demanded to relieve these problems in the commercial production.